Ambient gas-phase hydrogen peroxide (H2O2) and organic hydroperoxides (ROOHs) were measured at Mace Head Atmospheric Monitoring Station during September 1998 and June 1999. These measurements formed part of the New Particle Formation and Fate in the Coastal Environment (PARFORCE) campaign with the intention of assisting with the elucidation of potential oxidation pathways that lead to the production of new particles. A reflux concentrator sampled ambient air prior to on-site analysis by a peroxidase fluorescence technique with specific peroxide detection. Hydrogen peroxide, methyl hydroperoxide (CH3OOH), and several hydroperoxide species were detected. The highest H2O2 concentrations were observed during periods of clean maritime air, and a significant correlation exists between H2O2 concentration and tidal cycle during days when air masses had advected over inter tidal zones upwind of Mace Head. Peak H2O2 concentrations were observed around low tide on these days, coincident with particle events.